Prepregs of curable epoxy resin compositions with 9,9-Bis (4-aminophenyl) fluorenes as curatives

ABSTRACT

Curable epoxy resin compositions, preferably solvent-free, wherein a fluorene amine curative is partially melt dissolved and partially dispersed as a solid in at least one aromatic polyepoxide. The compositions provide prepregs which exhibit tack with surprisingly good shelf life properties. Cured composites comprising the prepregs are also provided. The cured composites exhibit little resin migration and glass transition temperatures that are comparable to those of cured neat resins.

This is a divisional of application Ser. No. 08/532,941 filed Sep. 22,1995, now U.S. Pat. No. 5,728,755, which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to epoxy resin compositions, prepregs,cured composites, and methods of making the same. More specifically,this invention relates to a curable epoxy resin composition preparedfrom an aromatic polyepoxide and a fluorene amine curative. The fluoreneamine curative is partly melt-dissolved and partly dispersed as a solidin the aromatic polyepoxide. Preferably, the curable epoxy resincomposition is solvent-free. Prepregs prepared from the curable epoxyresin composition advantageously exhibit tack in a curable state, withsurprisingly good shelf life properties. Furthermore, compositesprepared from prepregs of the invention advantageously cure uniformlywith minimal resin migration and exhibit glass transition temperatures(Tgs) comparable to those of the corresponding cured neat resins.

BACKGROUND OF THE INVENTION

Fiber reinforced composites are rapidly emerging as a primary materialfor use in high performance applications such as manufacture of aircraftcomponents. Fiber reinforced composites provide structural efficiency atlower densities compared to metallic structures, allowing for themanufacture of light weight, high strength components. Fiber reinforcedcomposites may be prepared using a variety of techniques, for example,hand or automated layup of prepreg, filament winding, compressionmolding and resin transfer molding. Of these techniques, hand orautomated layup of prepreg is most common.

A prepreg comprises a fiber reinforcement impregnated with an uncured orpartially cured resin matrix. Prepregs are available in a variety offorms depending on the configuration of the fiber reinforcement. Forexample, when the fiber reinforcement comprises a fiber bundle (or tow),the prepreg is specifically referred to as a "towpreg". By way ofanother example, when the fiber reinforcement comprises a collimatedseries of fiber bundles, the prepreg is specifically referred to as"prepreg tape".

Prepregs are typically supplied to part fabricators who convert thematerial into cured composite components using heat and pressure to curethe resin. For example, when the prepreg is in the form of a tape, thepart fabricator cuts lengths of the tape and places them on a toolsurface in the desired ply orientation. This operation can be donemanually or automatically and is generally referred to as "layup". Whenthe tool has a complex or curved or vertical configuration, the prepregpreferably has good tack to hold the plies together and to the tooluntil layup is complete. The prepreg also preferably has good drape orconformability, allowing it to conform to the tool shape. Preferably,the prepreg cures uniformly to provide composite parts having high glasstransition temperatures. This allows the cured composite to withstand avariety of stresses (such as elevated temperatures, mechanical stresses,exposure to solvents, etc.) without loss of structural integrity.

Epoxy resin compositions can be used as the resin matrix for prepregs.Several references describe epoxy resin compositions comprising fluoreneamine curatives. U.S. Pat. No. 5,276,106 (Portelli et al.) describes athermosettable epoxy resin composition prepared by dispersion ofthermoplastic particles, curatives, hardeners, catalysts, and modifyingadditives into the epoxy resin at a temperature at which thethermosettable resin is liquid, generally at about 30° C. to 60° C. U.S.Pat. No. 4,684,678 (Schultz et al.) describes a thermally curable epoxyresin composition prepared by mixing aromatic polyepoxides and curingagent or agents and/or catalysts to form a substantially uniformmixture. WO 95/05411 (Hardy et al.) describes a thermally curablearomatic amine-epoxy composition prepared by combining a polyepoxy, apolyamine, and a cure accelerator with mixing until the solids areuniformly distributed.

While these compositions have proven useful for a variety ofapplications, a need still exists for an epoxy resin compositioncomprising fluorene amine curative that can be used to provide prepreghaving tack, suitable viscosity characteristics even after aging, andwhich uniformly cures to provide cured composites that exhibit glasstransition temperatures comparable to those of the corresponding curedneat resins (i.e., resins prepared without fiber reinforcement).Preferably, such an epoxy resin composition is solvent-free forenvironmental reasons and to preclude the presence of residuals whichcan cause porosity during cure, possibly resulting in reducedperformance characteristics.

SUMMARY OF THE INVENTION

The present invention provides a curable epoxy resin compositioncomprising at least one aromatic polyepoxide and at least one fluoreneamine curative. Preferably, the curable epoxy resin composition issolvent-free. A portion of the fluorene amine curative is melt dissolvedin the aromatic polyepoxide, while the remainder is dispersed as a solidin the aromatic polyepoxide. As used herein, the term "melt dissolved"means that the fluorene amine curative and aromatic polyepoxide areheated sufficiently so that the fluorene amine curative dissolves togive a homogenous, single phase resin. As used herein, the term"dispersed" means that the fluorene amine curative is present in thearomatic polyepoxide as a dispersed, undissolved solid such as a powder.

The present invention also provides a prepreg comprising a fiberreinforcement impregnated with the curable epoxy resin compositiondescribed above. Prepregs comprising epoxy resin compositions havingboth melt dissolved and dispersed fluorene amine curative advantageouslyprovide tack at room temperature (21-25° C.). Tackiness wasqualitatively evaluated using the procedure described in the examples.Briefly, two pieces of prepreg were overlapped with the application ofpressure, and the resistance to separation was qualitatively assessed.

In addition to providing tack, prepregs of the invention surprisinglyprovide excellent shelf life properties despite the expectation that thepresence of melt dissolved fluorene amine curative would, over time,cause the epoxy resin to prematurely advance, resulting in an increasein resin viscosity. For purposes of the present invention, shelf lifewas evaluated by comparing the viscosity profile as a function oftemperature of a newly prepared epoxy resin composition against that ofa room temperature (21-25° C.) aged composition. Preferably, theviscosity profile showed little change with aging. More preferably, theminimum viscosity of the viscosity profile remained in the range of 0.3to 30 poise for the aged composition. This allows for the provision of aprepreg which can be layed up on a tool and allowed to sit at roomtemperature for a length of time (for example, 60 days) prior to curewithout the need for special storage conditions.

Preferably, a sufficient amount of the fluorene amine curative is meltdissolved to provide a prepreg that is tacky at room temperature, i.e.,has an initial resin glass transition temperature (Tg) less than orequal to 15° C. More preferably, the prepreg has an initial resin Tg inthe range of from -5° C. to 10° C., most preferably in the range of from0° C. to 10° C. It is also preferred that the amount of fluorene aminecurative that is melt dissolved be sufficient to provide uniform curewithout visible signs of resin migration. Resin migration occurs whenthe dispersed fluorene amine solid becomes trapped or filtered out bythe fiber reinforcement to such an extent that the cure stoichiometry isdisrupted, thereby causing portions of the composition to insufficientlycure. The insufficiently cured portions of the composition can migrateto the surface of the cured composite upon exposure to temperaturesgreater than the Tg of the cured composite. Cured composites whichexhibit resin migration are believed to have less structural integrityfor high performance applications (i.e., applications where thecomposite is exposed to elevated temperatures) and to be moresusceptible to attack by solvents.

At the same time, the amount of melt-dissolved fluorene amine curativeis preferably not so large that the prepreg becomes brittle, loses tackproperties, and can no longer be used in most prepreg applications wherethe prepreg must conform to the shape of a tool fixture or mandrel.

Within the broad considerations given above, the amount of fluoreneamine curative that is melt dissolved with the aromatic polyepoxide ispreferably in the range of from 10 to 90 percent by weight based on thetotal amount of fluorene amine curative. More preferred ranges depend onthe selection of the particular aromatic polyepoxide but generally arein the range of from 30 to 80 percent by weight, most preferably in therange of from 40 to 70 percent by weight. The remaining amount offluorene amine curative is dispersed in the aromatic polyepoxide insolid form, preferably as a powder.

The present invention also provides a method of making a prepregcomprising the steps of providing at least one aromatic polyepoxide;providing at least one fluorene amine curative; melt dissolving aportion of the fluorene amine curative into the aromatic polyepoxide;dispersing the remaining portion of the fluorene amine curative into thearomatic polyepoxide to form a resin matrix; providing a fiberreinforcement; and impregnating the fiber reinforcement with the resinmatrix.

The present invention further provides cured composites prepared fromprepregs of the invention. The cured composites of the inventionadvantageously exhibit uniform cure and Tgs that are comparable to(i.e., within ten degrees C. of) those of the corresponding cured neatresins. As used herein, the term "corresponding cured neat resin" meansthe same cured resin prepared without reinforcement. The curedcomposites can be used as structural and non-structural aircraftcomponents, space structures, pressure vessels, tanks, pipes, compositesfor electronics such as circuit boards, and automotive body and engineparts, and the like.

DETAILED DESCRIPTION OF THE INVENTION

The curable epoxy resin composition of the invention comprises at leastone aromatic polyepoxide and at least one fluorene amine curative.Preferably, the curable epoxy resin composition is solvent-free. Aportion of the fluorene amine curative is melt dissolved in the aromaticpolyepoxide, while the remainder is dispersed as a solid in the aromaticpolyepoxide. The prepreg of the invention comprises a fiberreinforcement impregnated with the curable epoxy resin composition. Weturn now to a discussion of aromatic polyepoxides, fluorene aminecuratives, and fiber reinforcements suitable for use in the presentinvention.

Aromatic Polyepoxide

Polyepoxides are compounds comprising at least two 1,2-epoxide groups,i.e., groups having the structure: ##STR1## Aromatic polyepoxides aredesired because they can impart high temperature performance properties(e.g., a high glass transition temperature) to the cured composite andcan impart structural properties thereto.

Aromatic polyepoxides suitable for use in the prepreg of the inventioninclude polyglycidyl ethers of polyhydric phenols, for example,pyrocatechol, resorcinol; hydroquinone;4,4'-dihydroxy-3,3'-dimethyldiphenyl methane; 4,4'-dihydroxydiphenylmethane; 1,1-bis(4-hydroxyphenyl)ethane; 1,2-bis(4-hydroxyphenyl)ethane;2,2-bis(4-hydroxyphenyl)propane; 4,4'-dihydroxydiphenyl cyclohexane;2,2-bis(3-methyl-4-hydroxyphenyl)propane; 4,4'-dihydroxydiphenylsulfone; tris-(4-hydroxyphenyl)methane; 9,9-bis(4-hydroxyphenyl)fluoreneand ortho-substituted analogues thereof, such as those disclosed in U.S.Pat. No. 4,707,534. Suitable aromatic polyepoxides also include thepolyglycidyl ethers of the halogenation (e.g., chlorination andbromination) products of the above-mentioned polyhydric phenols.

Other suitable aromatic polyepoxides include the polyglycidylderivatives of aromatic amines (i.e., glycidylamines) obtained from thereaction between the aromatic amines and an epihalohydrin. Examples ofsuch glycidylamines include N,N-diglycidyl aniline;N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane;N,N-diglycidylnapthalenamine (given the name of N-1napthalenyl-N-(oxiranylmethyl)oxiranemethanamine by Chemical Abstracts9th Coll. 8505F (1979-1982));N,N,N'N'-tetraglycidyl-1,4-bis[α-(4-aminophenyl)-α-methylethyl]benzene;andN,N,N',N'-tetraglycidyl-1,4-bis[α-(4-amino-3,5-dimethylphenyl)-.alpha.-methylethyl]benzene.The polyglycidyl derivatives of aromatic aminophenols (e.g.,glycidylamino-glycidyloxy benzene), as described in U.S. Pat. No.2,951,825, are also suitable. An example of these compounds isN,N-diglycidyl-4-glycidyloxybenzenamine.

Polyglycidyl esters of aromatic polycarboxylic acids, for example, thediglycidyl esters of phthalic acid, isophthalic acid, or terephthalicacid, are also useful.

Preferred aromatic polyepoxides include the polyglycidyl ethers of4,4'-dihydroxydiphenyl methane (bisphenol F),2,2-bis(4-hydroxyphenyl)propane (bisphenol A), and9,9-bis(4-hydroxyphenyl)fluorene.

Examples of commercially available aromatic polyepoxides include theAraldite™ series of materials such as MY-720, 721, 722, 0510, and 0500,and PY-306 and 307 (all available from Ciba-Geigy, Inc., Hawthorne,N.Y.); the EPON™ series of materials such as DPL-862 and HPT-1079 (ShellChemical Co., Houston, Tex.); and the D.E.R.™, D.E.N.™, and QUATREX™families of materials (Dow Chemical Co., Midland, Mich.).

While solid aromatic polyepoxide resins may be used, it is preferredthat the polyepoxide or mixture of polyepoxides be essentially liquid atroom temperature, by which it is meant that the viscosity of thepolyepoxide (or polyepoxide mixture) permits mixing and then spreading(e.g., coating) at room temperature, or upon gentle warming to atemperature that does not risk premature reaction of the curative (e.g.,room temperature to about 110° C.). Liquid aromatic polyepoxidesfacilitate mixing and spreading or coating of the resin composition atlow temperatures that do not activate the curative.

Preferably the aromatic polyepoxide (or polyepoxide mixture) has anaverage epoxide functionality of two to four, and, more preferably, anaverage epoxide functionality of two to three. This facilitatesproviding both an epoxy resin composition that can be mixed and coatedwithout premature reaction of the heat-activated curative, and a finalcured composite that is sufficiently crosslinked. It is also preferredthat the aromatic polyepoxide (or polyepoxide mixture) have an averageepoxy equivalent weight of about 80 to 200 grams per equivalent. Thispromotes the formation of epoxy resin compositions having a viscositythat permits efficient mixing and coating, and a final cured compositewith an acceptably high glass transition temperature.

Fluorene Amine Curative

The curing agent comprises at least one 9,9-bis(aminophenyl)fluorene,the phenyl and fluorene groups of which can be unsubstituted orsubstituted by one or more atoms or groups that are inert to reactionwith an epoxide group. Preferably, the fluorene amine curing agent hasthe following formula: ##STR2##

Each R³ is independently selected from hydrogen and groups that areinert to the polymerization of epoxide group-containing compounds.Preferably R³ is hydrogen, a halogen, a linear or branched alkyl grouphaving 1 to 6 carbon atoms, an aromatic group, a nitro group, an acetylgroup, or a trimethylsilyl group. R¹ and R² are independently selectedfrom hydrogen and linear and branched alkyl groups having 1 to 6 carbonatoms. Each R⁴, R⁵, R⁶ and R⁷ is independently selected from hydrogen,aromatic groups, the halogens, and linear and branched alkyl groupshaving 1 to 6 carbon atoms.

Examples of curing agents that satisfy the above general formula are:

9,9-bis(4-aminophenyl)fluorene,

4-methyl-9,9-bis(4-aminophenyl)fluorene,

4-chloro-9,9-bis(4-aminophenyl)fluorene,

2-ethyl-9,9-bis(4-aminophenyl)fluorene,

2-iodo-9,9-bis(4-aminophenyl)fluorene,

3-bromo-9,9-bis(4-aminophenyl)fluorene,

9-(4-methylaminophenyl)-9-(4-ethylaminophenyl)fluorene,

1-chloro-9,9-bis(4-aminophenyl)fluorene,

2-methyl-9,9-bis(4-aminophenyl)fluorene,

2-fluoro-9,9-bis(4-aminophenyl)fluorene,

1,2,3,4,5,6,7,8-octafluoro-9,9-bis(4-aminophenyl)fluorene,

2,7-dinitro-9,9-bis(4-aminophenyl)fluorene,

2-chloro-4-methyl-9,9-bis(4-aminophenyl)fluorene,

2,7-dichloro-9,9-bis(4-aminophenyl)fluorene,

2-acetyl-9,9-bis(4-aminophenyl)fluorene,

2-methyl-9,9-bis(4-methylaminophenyl)fluorene,

2-chloro-9,9-bis(4-ethylaminophenyl)fluorene, and

2-t-butyl-9,9-bis(4-methylaminophenyl)fluorene.

In more preferred fluorene amine curing agents, R¹, R², R³, R⁴, R⁵, R⁶and R⁷ are all as defined above, with the proviso that at least one ofR¹ and R² is a linear or branched alkyl group having 1 to 6 carbonatoms. Examples of such curing agents include:

9,9-bis(4-methylaminophenyl)fluorene,

9-(4-methylaminophenyl)-9-(4-aminophenyl)fluorene,

9,9-bis(4-ethylaminophenyl)fluorene,

9-(4-ethylaminophenyl)-9-(4-aminophenyl)fluorene,

9,9-bis(4-propylaminophenyl)fluorene,

9,9-bis(4-isopropylaminophenyl)fluorene,

9,9-bis(4-butylaminophenyl)fluorene,

9,9-bis(3-methyl-4-methylaminophenyl)fluorene,

9,9-bis(3-chloro-4-methylaminophenyl)fluorene,

9-(4-methylaminophenyl)-9-(4-ethylaminophenyl)fluorene,

9,9-bis(3,5-dimethyl4-methylaminophenyl)fluorene,

9-(3,5-dimethyl-4-methylaminophenyl)-9-(4-methylaminophenyl)fluorene,

1,5-dimethyl-9,9-bis(3,5-dimethyl-4-methylaminophenyl)fluorene

4-methyl-9,9-bis(4-methylaminophenyl)fluorene,

4-chloro-9,9-bis(4-methylaminophenyl)fluorene, and

9,9-bis(3,5-diethyl-4-methylaminophenyl)fluorene.

In the most preferred fluorene amine curing agents R³ is as definedabove, R¹ and R² are both hydrogen, and R⁴, R⁵, R⁶ and R⁷ areindependently selected from hydrogen, the halogens, aromatic groups, andlinear and branched alkyl groups having 1 to 6 carbon atoms but with thefurther provisos that at least one of the R⁴ and R⁵ moieties and atleast one of the R⁶ and R⁷ moieties are linear or branched alkyl groupshaving 1 to 6 carbon atoms, halogens, or an aromatic group.

Examples of the most preferred curing agents include:

9,9-bis(3-methyl-4-aminophenyl)fluorene,

9,9-bis(3-ethyl-4-aminophenyl)fluorene,

9,9-bis(3-phenyl-4-aminophenyl)fluorene,

9,9-bis(3,5-dimethyl-4-aminophenyl)fluorene,

9-(3,5-diethyl-4-aminophenyl)-9-(3-methyl-4-aminophenyl)fluorene,

9-(3-methyl-4-aminophenyl)-9-(3-chloro-4-aminophenyl)fluorene,

9,9-bis(3,5-diisopropyl-4-aminophenyl)fluorene, and

9,9-bis(3-chloro-4-aminophenyl)fluorene.

Preferred curatives also exhibit latent thermal reactivity, that is,they react primarily at higher temperatures (preferably temperatures ofat least 150° C.). This allows the epoxy resin composition to be readilymixed and coated at room temperature (about 21-25° C.) or with gentlewarming without premature reaction of the curative.

The curative is employed in an amount that is effective for providingthe desired high temperature performance properties in the curedcomposite. The actual amount of curative employed will also beinfluenced by the types and amounts of other components in the mixture.The fluorene amine curative is typically present in an amount sufficientto provide 1.0 to 2.0 moles of amino-hydrogen groups (NH) per mole ofepoxide groups. More preferably, the fluorene amine curative is presentin an amount sufficient to provide 1.2 to 1.65 moles NH groups per moleof epoxide groups. When the amount of curative falls significantlyoutside of these ranges, the final cured composite may have a low glasstransition temperature, a high coefficient of thermal expansion, reducedsolvent resistance, may absorb too much moisture, or may be brittle.

The fluorene amine curing agent can be supplemented with conventionalepoxy resin curing agents. Included among such supplementary curingagents are aliphatic and aromatic primary and secondary amines, forexample,4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, and2,2-bis(4-aminophenyl)propane; aliphatic and aromatic tertiary aminessuch as dimethylaminopropylamine and pyridine; imidazoles such as2-ethyl-4-methylimidazole; hydrazides such as adipodihydrazide;guanidines such as tetramethyl guanidine and dicyandiamide; andbenzyldimethyl amine.

Also useful as supplementary curing agents are Lewis acids such asaluminum chloride, aluminum bromide, boron trifluoride, antimonypentafluoride, phosphorous pentafluoride, titanium tetrafluoride and thelike. It is also desirable at times that these Lewis acids be blocked toincrease the latency of compositions containing them. Representative ofblocked Lewis acids are boron trifluoride complexes such as BF₃-diethylether, BF₃ -monoethanolamine, BF₃ -monoethylamine; and adductsof HSbF₅ X with either aniline-functional materials or a hindered amine,X being OH, halogen, or OR⁸ (R⁸ being an aliphatic or an aromaticgroup).

In addition, catalysts such as those described in WO 95/05411 may beused.

Fiber Reinforcement

The purpose of the fiber reinforcement is to provide strength to thecured composite. The fibers of the fiber reinforcement can comprise avariety of different materials including glass fibers, carbon fibers,polyamide fibers such as poly(p-phenylene terephthalamide) fibers (forexample, Kevlar™ fiber available from E.I. duPont de Nemours and Co.,Inc., Wilmingtom, Del.) and ceramic fibers. Carbon fibers are typicallyused as the reinforcing fiber in advanced aerospace structuralcomposites.

The fiber reinforcement may comprise a variety of configurations. Forexample, the fiber reinforcement may comprise a woven structureconstructed by interlacing yarns, fibers or filaments to form patternssuch as plain, harness satin or leno weaves. Alternatively, the fiberreinforcement may comprise a nonwoven structure or planar textilestructure produced by loosely compressing together fibers, yarns, andthe like. The fiber reinforcement may also comprise a tow (i.e., anuntwisted bundle of continuous fibers) or a roving (i.e., a number ofyarns, strands, tows or ends collected into a parallel bundle withlittle or no twist).

The fibers of the reinforcement may be unsized or coated with sizing.Preferably, the fibers are unsized. When a sizing is used, however, itpreferably does not materially affect the performance of the ultimateprepreg or cured composite, for example, by causing a substantialreduction in Tg.

Epoxy Resin Composition

The curable epoxy resin composition can be prepared by melt dissolving aportion of the fluorene amine curative with some or all of the aromaticpolyepoxide to form a homogenous, single phase blend. This can be doneusing, for example, an extruder or a heated mixer. Typically during thisstep, the epoxy resin composition is heated to about 149° C. (300° F.)for a time sufficient to allow dissolution (usually about fifteenminutes), but not so long that substantial curing of the epoxy resincompositon can occur.

The homogenous, single phase blend is then typically cooled slowly or byquench cooling, and the remaining polyepoxide (if any) and adjuvants arethen usually added. The remaining portion of fluorene amine curative isthen dispersed in the form of a solid, preferably as a powder, typicallyat temperatures less than about 49° C. (120° F.) using a high shearmixer or extruder.

As a variation of the above procedure, it is possible to melt dissolve aportion of the fluorene amine curative in a first fraction of aromaticpolyepoxide while uniformly dispersing the remaining fluorene aminecurative in a second fraction of aromatic polyepoxide, ultimatelybringing the first and second fractions together to form a uniform resincomposition.

A single extruder having multiple zones can be used to melt dissolve anddisperse the curative into the aromatic polyepoxide.

Preferably, a sufficient amount of the fluorene amine curative is meltdissolved to provide a prepreg that is tacky at room temperature, i.e.,has an initial Tg less than about 15° C. More preferably, the prepreghas an initial Tg in the range of from -5° C. to 10° C., most preferablybetween 0° C. and 10° C. It is also preferred that the amount offluorene amine curative that is melt dissolved be sufficient to avoidresin migration of insufficiently cured composition to the surface ofthe cured composite.

At the same time, it is preferred that the amount of melt-dissolvedfluorene amine curative is not so large that the prepreg becomesbrittle, loses tack, and can no longer be used in most prepregapplications where the prepreg must conform to the shape of a toolfixture or mandrel.

Within these parameters, the amount of fluorene amine curative that ismelt dissolved with the epoxy is preferably in the range of from 10 to90 percent by weight based on the total amount of fluorene aminecurative. More preferred ranges depend on the selection of theparticular aromatic polyepoxide but generally are in the range of from30 to 80 percent by weight, most preferably in the range of from 40 to70 percent by weight. The remaining amount of fluorene amine curative isdispersed in the aromatic polyepoxide in solid form.

Various adjuvants can also be added to the composition of the inventionto alter the characteristics of the cured composition. Included amonguseful adjuvants are thixotropic agents such as fumed silica; pigmentsor dyes; fillers such as silica, magnesium sulfate, calcium sulfate, andberyllium aluminum silicate; flame retardants; thermally conductiveparticles; electrically conductive particles; tackifiers; clays such asbentonite; solid or hollow spheres comprising glass, ceramic orpolymeric materials; and the like. Amounts of up to about 200 parts ofadjuvant per 100 parts of epoxy resin compositions can be used. Theadjuvants may be used alone or in combination.

Another adjuvant that may be used is a rubbery heterophase that isintroduced into the epoxy resin composition. For a detailed discussionof the use of a rubbery heterophase in epoxy resins, see Advances inChemistry Series, No. 208, titled "Rubber-Modified Thermoset Resins"edited by C. K. Riew and J. K. Gillham, American Chemical Society,Washington, 1984. Generally up to about 25 parts of rubbery phase per100 parts of epoxy resin compositions can be used.

Another useful adjuvant is a flow control agent. The purpose of the flowcontrol agent is to prevent loss of the resin due to flow during curing.Suitable flow control agents include thermoplastic resins, such aspolycarbonate, polysulfone, polyarylate, polyethersulfone,polyarylsulfone, polyester, polyetherimide, polyamideimide, polyimide,polyamide, or polyether resin, present as fine particles or dissolvedinto the resin matrix. Thermoplastic flow control agents are typicallypresent in an amount from 1 to 15 percent by weight based on the totalweight of the resin composition.

Prepreg Preparation

The curable epoxy resin composition can be used to impregnate a varietyof fiber reinforcements such as tows (i.e., bundles of fibers), or wovenstructures. Impregnation may be accomplished, for example, by heatingthe epoxy resin composition to temperatures at which it will flow(typically at a temperature of 110° C. or less) and depositing it ontothe fiber reinforcement. It is also possible to provide, for example, abath of flowing epoxy resin and immerse the fiber reinforcement (such astow) in the bath. Impregnation of the fiber reinforcement may also beaccomplished by forming a film of the epoxy resin composition on arelease liner and subsequently transfer laminating the film to a fiberreinforcement using pressure and/or heat. Preferably, for thislamination process, the curable epoxy resin composition has a viscosityin the range of from 10 to 30 poise at temperatures less than 110° C.for ease of processing and to provide sufficient wet out of the fibersof the reinforcement without initiating resin cure. Alternatively, thefiber reinforcement may be placed on a tool and then impregnated withthe resin composition by application of heat, pressure, and vacuum, orany combinations thereof. Methods for preparing prepregs employsolvent-free processing for environmental reasons and to preclude thepresence of residual volatiles which can cause porosity during cure,possibly resulting in reduced performance characteristics.

Prepregs of the invention provide tack and surprisingly good shelf lifeproperties. Tackiness was qualitatively evaluated using the proceduredescribed in the examples. Briefly, two pieces of prepreg wereoverlapped with the application of pressure, and the resistance toseparation was qualitatively assessed.

For purposes of the present invention, shelf life was evaluated bycomparing the viscosity profile as a function of temperature of a newlyprepared epoxy resin composition against that of a room temperature(21-25° C.) aged composition. Compositions were aged for up to 65 days.Preferably, the viscosity profile showed little change with aging. Morepreferably, the minimum viscosity of the viscosity profile remained inthe range of 0.3 to 30 poise for the aged composition. This allows forthe provision of a prepreg which can be layed up on a tool and allowedto sit at room temperature for a length of time (for example, 60 days)prior to cure without the need for special storage conditions. Theprepreg is then cured using heat and pressure such as provided byautoclave or press curing.

Resin compositions of the invention may be used to provide curedcomposites using a variety of processes such as pultrusion, filamentwinding, automated fiber placement, resin transfer molding, continuousresin transfer molding (CRTM™), resin film infusion, automated towplacement, automated and manual tape lay-up, vacuum bag lamination,press lamination, roll lamination, and the like.

Cured composites of the invention advantageously exhibit little to noresin migration and Tgs that are comparable to the corresponding neatresins. These characteristics enable the cured composite to withstand avariety of stresses (such as elevated temperatures, mechanical stresses,exposure to solvents, etc.) without loss of structural integrity.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples should not be construed to unduly limit thisinvention. In the examples, all parts and percents are by weight andtemperatures are in degrees centigrade, unless otherwise noted.

EXAMPLES General Preparation Methods General Preparation of ResinCompositions

The curable resin compositions were prepared by weighing the aromaticpolyepoxide component(s) into a 1.9 liter (0.5 gallon) metal can andheating it to 149° C. (300° F.) using a hot plate while stirring with anoverhead air stirrer fitted with a three-pronged mixing blade. A portionof the fluorene amine curative powder was added in a single charge withstirring at 149° C. (300° F.) until all the curative dissolved to give ahomogenous, single phase liquid resin. This liquid resin was thencharged (while still hot) into a planetary type mixer (commerciallyavailable as a Premier Mill Mixer from Premier Mill Corporation Temple,Pa.) which was at room temperature (approximately 21° C. (70° F.)). Thepolyepoxide/dissolved fluorene amine mixture was allowed to cool to 49°C. (120° F.) while stirring using a mixer speed setting of 3. Theremaining amount of fluorene amine curative powder was then added in asingle charge with stirring at 49° C. (120° F.) and application of avacuum of 70 cm (27 inches) Hg. Mixing was continued under theseconditions until all the powdered fluorene amine curative was uniformlydispersed as determined by the absence of visible "clumps" oragglomerates of powdered material.

General Preparation of Resin Transfer Films

Resin transfer films of the invention were prepared by coating the resincompositions onto a release liner having a width of 35.6 cm (14 inches).More specifically, the resin compositions were coated onto a 102 μm(0.004 inches) semi-bleached, silicone-coated paper release liner havinga basis weight of 33 kg/279 m² (72 lb/3000 ft) (commercially availableas "Stick-Not" from Release International, Chicago, Ill.) using a heatedreverse three-roll coater. The resin composition was heated for 15seconds in a 1200 Watt microwave oven (commercially available as ModelSpacemaker 2 from General Electric Corp.) operated at its highestsetting, removed, and briefly stirred by hand with a wood tonguedepressor. The resin composition was heated in this manner four to sixtimes to achieve a pourable viscosity for easy transfer; however,temperatures never exceeded 71° C. (160° F.). The gap between the twoheated rollers was set at 127 μm (0.005 inches) using a feeler gauge,and the temperature of the two heated rollers was set at 71° C. (160°F.). The resin was coated to give a coating weight of 0.01047 g/cm² anda coating width of 33.0 cm (13 inches). The resulting coated resintransfer films were then covered with a polyethylene liner (commerciallyavailable as Product No. GF-10R from Consolidated Thermoplastic,Chippewa Falls, Wis.) which was 35.6 cm (14 inches) wide and 76 μm(0.003 inches) thick, and wound onto a 7.6 cm (3 inches) diametercardboard core.

General Preparation of Prepreg

The coated resin transfer films were used to make resin-impregnatedcarbon fabric material, also referred to as "prepreg" material. Morespecifically, prepreg material was made from coated resin transfer filmsusing a hot melt prepregger (commercially available from CaliforniaGraphite Machines; Corona, Calif.). The polyethylene liners were removedfrom two rolls of a coated resin transfer film. The two exposed resinfilms were simultaneously laminated to opposite sides of a carbon fabricsubstrate comprising a fiber designated as G30-500-EPO1, having an 8harness satin weave, and an areal weight of 370 g/m² (commerciallyavailable as Product No. G105, Celion 3K, from Textile Technologies,Inc., Hatboro, Pa.) using heated nip rolls. The nip rolls were operatedusing a temperature setting of 95° C. (203° F.) and a pressure of 17.5kg/cm (98 lb/in). The line speed was 3 m/min (9.8 ft/min).

General Preparation of Cured Composites

The prepreg materials of the invention were used to make cured carbonfiber composites. More specifically, cured carbon fiber composites weremade by placing one ply of carbon fabric prepreg, 15.2 cm×15.2 cm (6inches×6 inches), on top of another, placing this two-ply layup on avacuum table covered with a release liner, and applying a vacuum of 71cm (28 inches) Hg at 25° C. (77° F.) for 5 minutes to remove airentrapped between the two plies and consolidate them. Two such two-plylayups were then placed one on another and consolidated as describedabove to give a four-ply layup. Finally, two such four-ply layups wereplaced one on another and consolidated as described above to give aneight-ply layup. The eight-ply layup was placed on the center of a metalplate [20 cm×20 cm×0.3 cm (8 inches×8 inches×0.12 inches)] which waswrapped with 0.01 cm (0.004 inches) thick Teflon™ release film(commercially available from Dewal Industries, Inc., Saunderstown,R.I.). Two 25 cm (10 inches) long glass fiber bleeder strings were thenplaced on the top surface of the eight-ply layup, each one 2.5 cm (1inch) from opposite outer edges of the eight-ply layup. The bleederstrings extended beyond the layup by 5 cm (2 inches) on both ends. Anadhesive-backed cork dam, 0.64 cm (0.25 inches) wide and 0.32 cm (0.13inches) thick, was placed around the eight-ply layup and adhered to thebottom plate so that the bleeder strings extended over the cork ribbon.A second metal plate [15.2 cm×15.2 cm×0.16 cm (6 inches×6 inches×0.06inches)] wrapped with the same 0.01 cm (0.004 inches) thick Teflon™release film described above was then placed on top of the eight-plylayup and rested inside the border created by the cork dam. The edges ofthe top metal plate were sealed with 50.8 mm (2 inch) wide 3MScotchmark™ Polyester Composite Bonding Tape having a thickness of 0.005cm (0.002 inches). The eight-ply layup was placed onto an autoclavetool, covered with a vacuum bag, and cured in an autoclave in aconventional manner. A vacuum of 76 cm (30 inches) Hg was applied for 15minutes at 25° C. (77° F.). A pressure of 0.59 MPa (85 psi) was thenapplied. During the pressure rise, the vacuum was released when thepressure reached 0.28 MPa (40 psi). Upon reaching the final pressure,the temperature was raised to 177° C. (351° F.) at a rate of 2°C./minute (3.6° F./minute). The layup was cured at this temperature forfour hours, after which the autoclave was internally cooled with amixture of steam and cold tap water to 25° C. (77° F.) at a rate of 5°C./minute (9° F./minute).

Test Methods Initial Resin Glass Transition Temperature (Tg)

A Model 912 dual cell differential scanning calorimeter (DSC) equippedwith a Thermal Analyst 2000 (both commercially available from TAInstruments, Inc., New Castle, Del.) was used to measure the Tg of thecurable resin compositions. The calorimeter was calibrated at 156.6° C.using an Indium standard. About 4 to 12 milligrams of the curable resincomposition was placed in a DSC pan, sealed shut, and placed in thesample cell of the DSC. A sealed empty pan and lid was placed in thereference cell. The sample was cooled to -50° C. with liquid nitrogen,then scanned from -50° C. to 50° C. at a rate of 10° C./minute under anitrogen purge. The Tg was taken as the midpoint of the observedtransition. Reported values are rounded to the nearest whole degree. ATg of less than or equal to 15° C. is desirable for providing prepregmaterials with sufficient tack to facilitate the layup of complex partsby either hand or machine.

Prepreg Tack

Prepreg materials were evaluated for their tack characteristics byoverlapping two pieces of fabric prepreg having dimensions of 7.6 cm (3inches) in length and 2.5 cm (1 inch) in width to form an overlap regionof 2.5 cm (1 inch) along the length of each material. A 160 gram rubberroller was rolled across the overlapping area four times to press thepieces together. This overlapping 2-ply layup was then pulled apart inthe length direction (shear mode) using one hand on each non-overlappedend. The resistance to separation was qualitatively determined andassigned a relative rating on the following scale:

Good: the two pieces stuck together and separated with significantresistance;

Slight: the two pieces stuck together and separated with someresistance;

Poor: the two pieces stuck together but separated with very littleresistance;

None: the two pieces did not stick to each other at all.

Prepregs rated "Good" or "Slight" are expected to be suitable forautomated or hand layup of composite parts. Prepregs rated "Good" areexpected to be especially suitable for automated processes, includingthose processes where composites having complex shapes (e.g., non-linearshapes) are desired. Prepregs rated "Poor" are expected to be suitablefor hand layup only of parts. Prepregs rated "None" are expected to beunsuitable for automated processes or hand layup of composite partshaving complex shapes, but probably could be used to provide hand layupof very simple, linear or flat parts.

Curable Resin Viscosity Minimum

A Rheometrics RDA-II Dynamic Mechanical Analyzer (commercially availablefrom Rheometrics Inc., Piscataway, N.J.) was used in the parallel platemode of operation to measure the minimum viscosity of the curable resincompositions. Three to five grams of curable resin composition wereplaced between the parallel plates (bottom plate diameter=50millimeters, top plate diameter=40 millimeters). The resin was loaded at25° C. (77° F.) for Examples 1-4, 9-11 and Comparative Examples C1 andC4; at 40° C. (104° F.) for Examples 5, 6 and 12; and at 50° C. (122°F.) for Examples 7, 8 and 13 and Comparative Examples C2 and C3. Theplates were then closed to provide a 1.0 millimeter gap filled withresin. Excess resin was scraped from the edges with a razor blade. Eachsample was equilibrated for fifteen minutes by applying a torquefrequency of 100 radians/second and a strain of 10% at a temperature of32° C. (90° F.). Each sample was then heated at 2° C./minute (3.6°F./min) to 179° C. (354° F.) and held at that temperature until aviscosity increase was observed. The viscosity was recorded every minuteon a dual-y axis plot as a function of temperature. The left y-axis ofthe plot provided viscosity in poise; the right y-axis of the plotprovided temperature, and the x-axis provided time. The minimumviscosity (i.e., the lowest viscosity point of the profile) was recordedPreferably, the minimum viscosity of the viscosity profile remained inthe range of 0.3 to 30 poise for the aged composition. This allows forthe provision of prepreg which can be layed up on a tool and allowed tosit at room temperature for a length of time prior to cure to form acured composite without the need for special storage conditions.

Cured Composite Laminate Glass Transition Temperature (Tg)

The Tg of the cured composite laminates was measured by dynamicalmechanical analysis using the same Rheometrics RDA-II Dynamic MechanicalAnalyzer described above. A rectangular sample measuring about 1.3 cm inwidth by about 5.1 cm in length by about 0.16 to 0.48 cm in thickness(0.5×2.0×(0.06-0.19) inches)) was cut from the cured composite andplaced between the upper and lower grips of the analyzer. The analyzerwas equipped with an oven that heated the sample from 50° C. (122° F.)to 220° C. (428° F.) in 5° C. steps. The sample was held at the desiredtemperature for one minute before recording data. A sinusoidal torquewith a frequency of 10 radians/second was applied to the lower grip,which, in turn, applied a strain to the sample. The resultant stress wasmonitored by the upper grip and was recorded every 5° C. The recordeddata was used to calculate both storage modulus (G') and loss modulus(G"), with the onset of the observed transition, i.e., the inflectionpoint, of G' taken as the Tg. Reported values are rounded to the nearestwhole degree.

Cured Neat Resin Glass Transition Temperature (Tg)

The Tg of a cured neat resin was measured by dynamical mechanicalanalysis using the same Rheometrics RDA-II Dynamic Mechanical Analyzerdescribed above. Two aluminum plates [15.2 cm in length by 11.4 cm inwidth (6×4.5 inches)] were cleaned with a ScotchBrite™ pad (availablefrom 3M Co. St. Paul, Minn.) saturated methyl ethyl ketone (MEK)followed by a MEK wipe. The plates were then wiped with RAM 225 moldrelease agent (available from Lilly-Ram Industries, Alexandria, Ohio).The plates were used to prepare a mold by clamping a Teflon™ spacer [0.3cm (1/8 inches) thick] between the two plates. The Teflon™ spacer waspresent on three sides of the assembly, providing a 2.5 cm (1 inch)border of Teflon™. This entire assembly was placed in an oven at 177° C.(350° F.) for 30 minutes to cure the RAM 225 mold release.

50 g of the resin was placed in a 0.2 liter (0.5 pint) metal can andstirred at 149° C. (300° F.) until the resin changed from opaque toclear. The can was then placed in a vacuum oven heated to 121° C. (250°F.). A vacuum of 71 cm (28 inches) Hg was applied until the air wasremoved from the resin as demonstrated by the cessation of bubbling andfoaming. The resin was then poured into the hot mold and cured in anoven for 4 hours at 177° C. (350° F.). The mold assembly was removedfrom the oven and allowed to cool before the cured resin was removedfrom the mold. This cured neat resin was then tested for Tg using theprocedure described above for a cured composite laminate.

Resin Migration in Cured Composite

After measurement of the cured composite Tg, as described above, thesamples were visually examined for the presence of "shiny" spots whichare believed to be indicative of resin migration. A relative rating wasassigned as follows:

None no shiny spots were observed on either the surface or cut edges;

Low shiny spots were observed only on the surface; and

High shiny spots were observed on both the surface and cut edges.

Those samples given a "None" rating are expected to be especially usefulas cured composites for structural applications in the aerospaceindustry. Samples given a "Low" rating are expected to be suitable foruse as cured composites for other, less demanding applications such asnon-structural aircraft components, pressure vessels, tanks, pipes,composites for electronics such as circuit boards, and automotive bodyand engine parts, and the like. Cured composites given a "High" ratingare less desirable as these composites are believed to have lessstructural integrity and be more susceptible to attack by solvents.

Glossary

Various abbreviations are used in the following examples. Theabbreviations are defined according to the following schedule:

CAF--9,9-bis(3-chloro-4-aminophenyl)fluorene

FEP--the diglycidyl ether of 9,9-bis(4-hydroxyphenyl)fluorene (availableas EPON™ HPT 1079 from Shell Chemical Co., Houston, Tex.)

D.E.R.™332--diglycidyl ether of bisphenol A (available from Dow ChemicalCo., Midland, Mich.)

EPON™ DPL-862--diglycidyl ether of bisphenol F (available from ShellChemical Co.)

Examples 1-8

A series of curable resin compositions, resin transfer films, prepregs,and cured composites were prepared using the types and amounts ofcurative and polyepoxide as shown in Table 1 and the methods describedabove in "General Preparation Methods", except that in Examples 2, 4, 6,and 8, a 0.2 liter (0.5 pint) metal can was used to carry out thedissolution of the fluorene amine curative powder and only resincompositions were prepared. Samples were tested using the proceduresdescribed above. Test results are provided in Table 1.

Example 9

A curable resin composition, resin transfer film, prepreg, and curedcomposite were prepared using the types and amounts of curative andpolyepoxide as shown in Table 1 and the methods described above in"General Preparation Methods", except that a 3.8 liter (1.0 gallon) canwas used to carry out the dissolution of the fluorene amine curative.These materials were tested as described above, and the results areshown in Table 1.

Comparative Example 1 0% Melt Dissolved

A resin composition and resin transfer film were prepared using thetypes and amounts of curative and polyepoxide as shown in Table 1 andthe methods described above in "General Preparation Methods", exceptthat none of the powdered fluorene amine was dissolved in thepolyepoxide component. The structural integrity of the resin compositionwas too low to permit preparation of curable prepreg material fortesting or conversion to a cured composite. As a result, only theinitial resin Tg value was measured as shown in Table 1.

Comparative Example 2 100% Melt Dissolved

A curable resin composition, resin transfer film, prepreg, and curedcomposite were prepared using the types and amounts of curative andpolyepoxide as shown in Table 1 and the methods described above in"General Preparation Methods", except that all of the powdered fluoreneamine was dissolved in the polyepoxide component. The samples weretested using the procedures described above. Results are shown in Table1.

Comparative Example 3 100% Melt Dissolved

A resin composition was prepared using the types and amounts of curativeand polyepoxide as shown in Table 1 and the methods described above in"General Preparation Methods", except that all of the powdered fluoreneamine was dissolved in the polyepoxide component, a 0.4 liter (1 pint)metal can was used to carry out dissolution of the fluorene aminecurative powder, and only a resin composition was prepared. The testresults are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________           EPON.sup.TM                                                                             Dissolved   DSC        RDA Cured                                D.E.R.sup.TM DPL- Dissolved CAF (wt % Dispersed Initial Prepreg                                                          Composite Resin                   Ex. 332 (g) 862 (g) CAF (g) of total CAF) CAF (g) Resin Tg (°C.)                                                   Tack Tg (°C.)                                                          Migration                       __________________________________________________________________________    C1                                                                              554.9                                                                              0    0    0      445.2                                                                              -16.4  N/M*                                                                              N/M*  N/M*                              1 554.9 0 89.0 20 356.1 -12.5 None 176 Low                                    2 55.5 0 13.4 30 31.2 -9.6 N/M N/M N/M                                        3 554.9 0 178.1 40 267.1 -5.0 Poor 184 Low                                    4 55.5 0 22.3 50 22.3 -0.7 N/M N/M N/M                                        5 554.9 0 267.1 60 178.1 3.2 Good 183 None                                    6 55.5 0 31.2 70 13.4 7.1 N/M N/M N/M                                         7 554.9 0 356.1 80 89.0 10.7 Good 182 None                                    8 55.5 0 40.1 90 4.5 14.6 N/M N/M N/M                                         C2 554.9 0 445.1 100 0 17.5 None 188 None                                     9 0 1098.3 450.9 50 450.9 -3.3 Slight 149 High                                C3 0 109.8 90.2 100 0 18.1 N/M N/M N/M                                      __________________________________________________________________________     N/M: not measured (no prepreg was made)                                       *structural integrity of sample was too low to permit prepreg preparation     Note:                                                                         The RDA Tg for the cured neat resin corresponding to Examples 1-8, C1 and     C2 was 186° C., while the RDA Tg for the cured neat resin              corresponding to Examples 9 and C3 was 156° C.                    

Discussion of Results of Table 1

The results in Table 1 show that when the fluorene amine curative ispartly melt-dissolved and partly dispersed into the polyepoxide, theproperties of the prepreg and the cured composite surprisingly improvecompared to systems where the curative is either all dispersed or allmelt dissolved. The resin composition of C1 (0% melt dissolved) lackedsufficient structural integrity for preparation of a prepreg that couldbe tested or converted to a cured composite. C2 provided a prepreg thatwas brittle. In addition, the data corresponding to C2 and C3 show thatwhen the fluorene amine is completely dissolved, the tackcharacteristics of the prepreg are not acceptable as shown by glasstransition temperatures greater than 15° C.

As the amount of melt dissolved curative is increased, the problemassociated with C1 is overcome, and prepreg having improved tack withglass transition values of less than 15° C. is provided. Furthermore,the prepreg provides cured composites having a resin migration rating of"Low" to "None" and cured glass transition temperatures that arecomparable (i.e., within 10 degrees C.) to the cured neat resin glasstransition temperature.

From this data, it can be concluded that when the epoxy resincomposition comprises D.E.R.™ 332 and CAF, the preferred amount ofdissolved curative is in the range of from 40% to 80% to achieve adesirable combination of tack, cured laminate Tg, and resin migrationcharacteristics.

Examples 10-13

A series of curable resin compositions, resin transfer films, prepregs,and cured composite laminates were prepared using the types and amountsof curative and polyepoxide as shown in Table 2. As can be seen fromTable 2, the polyepoxide of these examples comprised a mixture ofD.E.R.™ 332 and FEP (a solid polyepoxide): The materials were preparedas follows:

FEP was first dissolved in the D.E.R.™ 332 prior to dissolving anyfluorene amine curative powder. This was done by weighing the twopolyepoxide components into a 3.8 liter (1.0 gallon) metal can andheating to 107° C. (225° F.) using a hot plate while stirring with anoverhead air stirrer fitted with a three-pronged mixing blade. Heatingand stirring was continued until a homogenous, single phase liquid resinwas obtained. The temperature of the D.E.R.™ 332/FEP mixture was thenincreased to 149° C. (300° F.) with continuous stirring, at which time aportion of the fluorene amine curative powder was added in a singlecharge with continuous stirring. The stirring was continued until allthe curative dissolved to give a homogenous, single phase liquid resin.The addition and dispersal of the remaining amount of fluorene aminecurative was conducted in the same manner as described above in the"General Preparation Methods, General Preparation of ResinCompositions". Resin transfer films, prepregs, and cured composites werealso prepared using the procedures described above in the "GeneralPreparation Methods". The compositions and test results for theseresins, prepregs, and composites are shown in Table 2.

Comparative Example 4 0% Melt Dissolved

A curable resin composition, resin transfer film, prepreg, and curedcomposite were prepared as described above for Examples 10-13 exceptthat none of the fluorene amine curative powder was dissolved. Thecomposition and test results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                     Dissolved   DSC        RDA Cured                                D.E.R.sup.TM  Dissolved CAF (wt % Dispersed Initial Prepreg Composite                                                    Resin                             Ex. 332 (g) FEP (g) CAF (g) of total CAF) CAF (g) Resin Tg (°C.)                                                   Tack Tg (°C.)                                                          Migration                       __________________________________________________________________________    C4                                                                              861.0                                                                              287.0                                                                              0    0      852.0                                                                              -5.7   Poor                                                                              187   High                              10 861.0 287.0 85.2 10 766.8 -5.5 Slight 190 High                             11 861.0 287.0 170.4 20 681.6 -0.3 Slight 188 Low                             12 861.0 287.0 255.6 30 596.4 5.4 Good 186 Low                                13 861.0 287.0 340.8 40 511.2 9.4 Good 190 Low                              __________________________________________________________________________     Note:                                                                         The RDA Tg of the cured neat resin corresponding to Examples 10-13 and C4     was 187° C.                                                       

Discussion of the Results of Table 2

The results in Table 2 show when a portion of the fluorene aminecurative powder is predissolved in the resin compositions, a desirablecombination of tack, cured laminate Tg, and resin migrationcharacteristics may be achieved. More specifically, prepreg tackimproves from "poor" to "slight" to "good" as the amount of meltdissolved curative is increased from 0% to 10% to 30%. The data alsoshow that when the epoxy resin composition comprises D.E.R.™ 332, FEP,and CAF, the amount of melt dissolved curative is preferably from 30 to40% to achieve a desirable balance of tack, cured laminate Tg, and resinmigration characteristics.

Aging Characteristics

The minimum viscosity of the curable resin compositions of Examples 3,6, 9 and 13 were evaluated using the procedure as described above at thetime they were prepared and several weeks later in order to evaluatetheir room temperature stability, i.e., aging characteristics. Theresults are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Example      Time    Viscosity Minimum                                          (#) (days) (Poise)                                                          ______________________________________                                        3            0       0.3                                                        3 65 0.4                                                                      6 0 0.8                                                                       6 28 2.2                                                                      9 0 0.4                                                                       9 46 2.8                                                                      13 0 0.4                                                                      13 35 0.8                                                                   ______________________________________                                    

The results in Table 3 show that when 40 to 70% of the fluorene aminecurative powder is predissolved in the resin composition it retains itsviscosity characteristics even after 65 days at room temperature. Thisallows for the provision of prepreg which can be layed up on a tool andallowed to sit at room temperature for a length of time (for example, 60days) prior to cure without any requirements for special storageconditions.

We claim:
 1. A prepreg comprising a fiber reinforcement impregnated witha curable epoxy resin composition, said epoxy resin compositioncomprising:(a) at least one aromatic polyepoxide compound having atleast two 1,2-epoxide groups; and (b) at least one9,9-bis(4-aminophenyl)fluorene amine curative, wherein:(i) at least 10%by weight of said fluorene amine curative is heat dissolved in saidaromatic polyepoxide, based on the total amount of fluorene aminecurative used; and (ii) at least a portion of said fluorene aminecurative is dispersed as a solid therein, wherein components (a) and(b)(i) form a homogenous single phase resin having an initial resinglass transition temperature of less than or equal to 15° C. as measuredby differential scanning calorimetry at a heating rate of 10° C. perminute.
 2. A prepreg according to claim 1 wherein said curable epoxyresin composition is solvent-free.
 3. A prepreg according to claim 1wherein the amount of fluorene amine curative that is heat dissolved insaid aromatic polyepoxide is in the range of from 10 to 90 percent byweight based on the total amount of fluorene amine curative used.
 4. Aprepreg according to claim 1 wherein the amount of fluorene aminecurative that is heat dissolved in said aromatic polyepoxide is in therange of from 30 to 80 percent by weight based on the total amount offluorene amine curative used.
 5. A prepreg according to claim 1 whereinthe amount of fluorene amine curative that is heat dissolved in saidaromatic polyepoxide is in the range of from 40 to 70 percent by weightbased on the total amount of fluorene amine curative used.
 6. A prepregaccording to claim 1 wherein the initial glass transition temperature ofcomponents (a) and (b)(i) of said epoxy resin composition is in therange of from -5° C. to 10° C. as measured by differential scanningcalorimetry at a heating rate of 10° C. per minute.
 7. A prepregaccording to claim 1 wherein the initial glass transition temperature ofcomponents (a) and (b)(i)of said epoxy resin composition is in the rangeof from 0° C. and 10° C. as measured by differential scanningcalorimetry at a heating rate of 10° C. per minute.
 8. A prepregaccording to claim 1 wherein said aromatic polyepoxide is selected fromthe group consisting of the polyglycidyl ethers of4,4'-dihydroxydiphenyl methane, 2,2-bis(4-hydroxyphenyl)propane, and9,9-bis(4-hydroxyphenyl)fluorene.
 9. A prepreg according to claim 1wherein said curative is present in an amount sufficient to provide 1.0to 2.0 moles of amino-hydrogen groups per mole of epoxide groups.
 10. Aprepreg according to claim 1 wherein said curative is present in anamount sufficient to provide to provide 1.2 to 1.65 moles ofamino-hydrogen groups per mole of epoxide groups.
 11. A prepregaccording to claim 1 wherein said epoxy resin composition furthercomprises at least one adjuvant.
 12. A prepreg according to claim 1wherein said epoxy resin composition has a minimum viscosity in therange of from 0.3 to 30 poise, as measured by a dynamic mechanicalanalyzer in a parallel plate mode using a torque frequency of 100radians per second and a heating rate of 2° C. per minute.
 13. A prepregaccording to claim 1 wherein said epoxy resin composition has a minimumviscosity in the range of from 10 to 30 poise at a temperature less than110° C. as measured by a dynamic mechanical analyzer in a parallel platemode using a torque frequency of 100 radians per second and a heatingrate of 2° C. per minute.
 14. A cured composite comprising the curedprepreg of claim
 1. 15. A cured composite according to claim 14 having aglass transition temperature that is within ten degrees Celsius of theglass transition of the corresponding cured neat resin, as measured by adynamic mechanical analyzer using a sinusoidal torque applied at afrequency of 10 radians per second and a heating rate of 5° C. stepswith a one minute hold at each step.
 16. A prepreg according to claim 11wherein the at least one adjuvant is selected from the group consistingof thixotropic agents, pigments, dyes, fillers, flame retardants,thermally conductive particles, electrically conductive particles,tackifiers, clays, flow control agents, rubbers, spheres of glass,ceramic materials, polymeric materials, and a combination thereof.